Icloud
How do I sign into someone else’s iCloud. I want to use their calendar
Question marked as
You don't.
iCloud accounts are personal and private. You should be logging in to someone else's because that would cause merging off all data in iCloud with yours. Calendar, messages, notes, contacts, photos etc... So no, signing into someone else's icloud account is not a good idea.
If you want to share a calendar you don't need to log in to their account. See here: iCloud: Share calendars in iCloud